An ischemic stroke occurs when the flow of blood to the brain is obstructed. Cerebral ischemia and reperfusion injury, due to diseases such as stroke and cardiac arrest, are a leading cause of disability and death in the US. Over 700,000 strokes occur annually in the United States (U.S.) alone, and, as of 2001, 4,800,000 people were living with the consequences of stroke. The overall death rate for stroke is approximately 58%, with approximately 50% of these patients dying in a hospital (American Heart Association Heart Disease and Stroke Statistics, 2004 Update).
In addition to ischemic stroke, hemorrhagic stroke (including intracerebral and subarachnoid types) account for 12% of all stroke incidents. Unlike ischemic strokes, hemorrhagic stroke is due to loss of blood from the vascular system into the parenchymal, subarachnoid or subdural space. This type of stroke incurs a much higher mortality rate (37-38% mortality within 30 days) in comparison to ischemic stroke. (American Heart Association Heart Disease and Stroke Statistics, 2004 Update). Hemorrhagic stroke often leads to the production of chronic delayed cerebral vasoconstriction and vasospasm. In fact, ⅔ of patients which have been treated for subarachnoid hemorrhage suffer from cerebral vasospasm between days 3 and 13 after hemorrhage. Suhardja, A., Nature Clin. Prac., Cardiovasc. Med. 1(2):110-116 (2004). These alterations in cerebral vascular function occur both in larger diameter vessels (e.g., basilar artery, middle cerebral artery), and in smaller diameter vessels, including capillaries and arterioles. Vasospasm is due in part to build up of reactive blood products surrounding the vasculature, including oxyhemoglobin, which sequesters the vasodilator nitric oxide (NO). Loss of cerebral blood flow autoregulation, including vasoconstriction and vasospasm, lead to increased cerebral damage from restricted blood flow, causing worsening of brain damage in the area surrounding the trauma, and accounting in part for the worsened outcome of hemorrhagic stroke patients.
Other causes of compromised cerebral vascular autoregulation include chronic high blood pressure, or hypertension. Hypertension affects approximately 1 in 5 Americans (1 in 4 adults) (American Heart Association Heart Disease and Stroke Statistics, 2004 Update) and is a significant risk factor for cerebral stroke injury, in addition to other chronic diseases including congestive heart failure. Hypertension has been correlated with reduced cerebral blood flow (Rodriguez, G., et al., Stroke 18(1):13-20 (1987)). Therefore, maintaining adequate cerebral perfusion is critical in managing cerebral protection in these at-risk patients. Control of cerebral blood flow is also important for maintaining brain perfusion in other idiopathic causes of cerebral vasoconstriction and vasospasm, including migraine, pregnancy, Call-Fleming syndrome and benign angiopathy of the central nervous system (Singhal, A. B. Top. Stroke Rehabil. 11(2),1-6 (2004)).
Additionally, the cost of cerebrovascular disease is enormous. Both from direct health expenditures and lost productivity due to morbidity and mortality, the cost of stroke in the U.S. amounts to approximately $53.6 billion. (American Heart Association Heart Disease and Stroke Statistics, 2004 Update).
Despite this large clinical need, current treatment options for cerebral ischemic and reperfusion injury are limited. The only clinically approved medication for stroke, recombinant tissue plasminogen activator (rtPA), is only used in about 1% to about 2% of patients nationally, due in large part to a short therapeutic window and high risk profile, including increased risk of cerebral hemorrhage and death.
In designing a drug or treatment for ischemic stroke, it is desirable to design a drug or treatment that may be administered multiple times without causing patient desensitization. Desensitization of patients to a drug occurs when the drug is administered and either has no therapeutic effect or the therapeutic effect decreases as a function of the number of times that the drug is administered. This occurs with several medications, including, for example, nitroglycerine used to treat cardiac ischemia.
There is therefore a need for a drug or treatment method for increasing blood flow in the blood vessels of the brain. There is further a need for such a drug or treatment method that does not cause patient desensitization, and therefore may allow chronic administration as needed. The present invention addresses these needs.